This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9803671-8 filed in Sweden on Oct. 26, 1998; the entire content of which is hereby incorporated by reference.
The present invention relates to a circuit board, in particular a multilayer circuit board of the type, which comprises at least a first carrying board section and a second board section and a conductor pattern and via holes.
The invention also refers lo a method to produce the circuit board.
To increase the number of components and circuits on a circuit board certain circuits can be mounted directly on the circuit board without protective housing. These so-called naked circuits are used frequently in special applications in exposed environments and must be protected against the surroundings, e.g. against moisture. The circuit is normally glued with silver glue, a process that can result in silver migration, which as a result of the moisture may become aggravated. Generally, they are provided with a protective cover of a material capable of being hardened, in particular the entire circuit board is covered with this material, which in addition can complicate fault-detecting and repair of the board.
In certain applications the naked circuits or chips can be packaged in a package containing both the circuit and a board. The drawback with this method is that the package requires an extra accommodation.
Prior art discloses only packaging of circuits for use within microwave applications on or under a circuit board. The circuits are then arranged with a protection of a cover or bottom.
U.S. Pat. No. 4,276,558 describes, for example, an active microwave amplifier, which is hermetical encased onto the back of a dielectric substrate, whose front is occupied by means for transmission/reception of signals. Hermetical sealing can occur when the circuit is provided with a lid with the same thermal characteristics as the substrate.
U.S. Pat. No. 5,229,727 describes a microstrip to microstrip junction through a hermetical scaling on a ceramic board.
Moreover, U.S. Pat. No. 5,712,767 discloses a circuit element mounting structure comprising, a substrate having a pattern A of wirings therein; at least one cavity formed at a surface of the substrate; and a cap for covering the cavity therewith, characterized by the cap including a pattern B of wirings therein and a connection B for electrically connecting the pattern B of wirings to the pattern A of wirings, and the substrate including a connection A for electrically connecting the connection B to the pattern A of wirings. In accordance with the invention, it is now possible to mount circuit elements on a cap which had been a dead space in prior circuit, and hence also possible to mount circuit elements in higher density on a substrate including a cap zone.
The cavities according to this invention are covered by means of covering the cavity with metal and then a metallized lid is soldered to the metal cover. The lids are arranged as electrical screening. Accordingly, each cavity is covered with a metallic cover, which results in time and cost consuming sealing process.
JP7022730 aims to reduce the occupying area in a composite electronic component including the combination of multilayer circuit boards and an electronic component chip. Therefore, a plurality of multilayer circuit boards having a cavity in which an electronic component chip is built an stacked in a condition of being mutually bonded to one other by a bonding agent. This enables the built-in electronic components to be discriminated between non-defective and defective at the pre-stacking stage of individual multilayer circuit boards.
The main object of this invention is to reduce the number of substrates carrying naked chips so if a chip is defective and the substrates must be discarded, not a few chips are discarded. To achieve an effective assembly, the substrates are stacked on each other aid the naked chips are located inside the cavities of the stack. Neither hermetic sealing nor moisture barriers are considered as plastic substrates are used.
The object of the present invention is to produce a new type of circuit board, comprising substrates made of not ceramic material, which allows much larger density of component than what has been possible. Non ceramic materials are generally much cheaper than ceramic material.
A second object of the invention is to allow hermetical sealing and integration of electric components, in particular naked chips to the board.
A third object of the invention is to be able by using the non ceramic board material obtaining essentially hermetical sealing of components.
A further object of the present invention is to achieve isolation of scaled components, in particular against penetration of moisture in transversal direction.
An advantage with the board according to the invention is that it can be arranged in a cooling medium without damage to the components in the hermetical sealed cavities.
These objects are obtained through the initially mentioned circuit board, which in at least one of the board sections includes at least one cavity for placing of at least one electric component, preferably a naked circuit. The second board section constitutes a protective cover for essentially hermetical sealing of the component.
In one embodiment, the section arranged as cover at least partly covers the first section.
The component provided in the cavity can suitably be connected to other components in other layers through the via holes.
Depending on the applications and components that are used the board comprises different cavities with different volumes. The number of components that can be mounted can be increased the second section, arranged as a cover, which can be arranged to provide further components.
To isolate the board against for instance penetration of moisture the edges of the sections are plated, which can be done before or after the assembly of the sections and a possible joint between the sections are adhering as moisture barrier.
Preferably one of the sections is provided with a groove while the other one is provided with corresponding elevation designed to, in cooperation with the grooves, provide a moisture barrier and/or assembly references.
In one embodiment the component in the cavity can be surfaces mounted.
To achieve same thermal expansion, the sections consist of a material with the same thermal characteristics, which can consist of woven glass fibre with adhesive of thermoplast. Suitably the adhesive contains ceramic filling.
The invention also comprises a method for manufacturing a circuit board, preferably a multilayer circuit board including at least a first carrying board section and a second board section and a conductor pattern and via holes. The method includes the primary steps of: producing laminate layers of a non ceramic material, producing via holes in the laminate layer, plating desired via holes, filling the via hole with a filler, providing a cavity in a laminate layer, providing desired laminate layer with a conductor layer or pattern, joining together the laminate layers, filling the holes with the filler through screen printing, removing possible section of laminate functioning as cover, and arranging a moisture barrier on the edge of said laminates. The method also comprises the steps of plating the edges of the layers. According to the method the components are arranged in the cavity and a laminate layer is arranged so that it covers the cavity.